Fish raised on the low-carotenoid diet had significantly lower concentrations of carotenoids in their body than high-carotenoid diet fish (effect of diet: F1,47=40.07, p<0.001; sex: F1,47=2.32, p=0.14; diet×sex interaction: F1,47=1.60, p=0.21; a). Furthermore, when we looked at the effect of diet on the expression of nuptial coloration, we found that males on the low-carotenoid diet had significantly reduced standardized coloration scores, measured from photographs (high-carotenoid males: 0.75±0.01; low-carotenoid males: 0.71±0.01; t29=2.97, p=0.006), and tended to deposit less carotenoids in their ornament than high-carotenoid males, although the difference between the two groups was not statistically significant (t29=1.81, p=0.081; a).
Figure 1 (a) Mean+s.e. total concentration of carotenoids in the body of males and females (hashed bars) and the nuptial coloration of males (black bars) and (b) mean+s.e. susceptibility to oxidative stress (measured in terms of MDA, a by-product of lipid peroxidation) (more ...)
The concentration of carotenoids contained in the area of nuptial coloration outweighed body carotenoid concentrations by a factor of 15.8, on average (a), suggesting that the maintenance of nuptial coloration may impose considerable costs. We therefore explored whether low-carotenoid males were paying a higher price in an attempt to maintain their sexual coloration. Consistent with this, males on the low-carotenoid diet invested a significantly greater proportion of their total pool of carotenoids (the sum of carotenoids in the sexual signal and body) in nuptial coloration (80.7±5.5%) when compared with high-carotenoid males (20.9±2.6%; t29=10.45, p<0.001).
Susceptibility to oxidative stress depended on dietary carotenoid intake, but this effect differed between the sexes (effect of diet: F1,47=7.51, p=0.009; sex: F1,47=10.48, p=0.002; diets×sex interaction: F1,47=4.10, p=0.049; b). In accordance with the hypothesis that low-carotenoid availability can increase an individual's susceptibility to oxidative stress and that there is a trade-off between signalling and antioxidant protection, post hoc tests revealed that males on the low-carotenoid diet showed a significantly greater susceptibility to oxidative stress than either high-carotenoid males or females, or low-carotenoid females. None of the other groups differed in their susceptibility (b).
Low-carotenoid males completed significantly fewer breeding rounds than high-carotenoid males (Mann–Whitney test: w17=217.5, p=0.005; a). In addition, while there was no effect of initial body weight on longevity (Cox regression: Χ12=0.05, p=0.82), males on the low-carotenoid diet had significantly shorter lifespans than those on the high-carotenoid diet (Χ12=9.56, p=0.002; b) with the great majority (93%) of low-carotenoid diet males dying during the breeding season when compared with only about half (54%) of the high-carotenoid diet males (b). Furthermore, a male's lifespan was significantly predicted by its standardized coloration score (ANCOVA, diet treatment group: F1,31=8.59, p=0.006; coloration score: F1,31=6.75, p=0.014; ), with redder males living longer than less red males.
Figure 2 (a) Distribution of breeding rounds completed by males on the high- (high-C) and low- (low-C) carotenoid diets (the median is indicated by the thick line), and (b) survival curves for males in the high- (filled circles and solid line) and low- (open circles (more ...)
Figure 3 The relationship between signal redness and remaining lifespan for males on the high- (filled circles and solid line) and low- (open circles and dashed line) carotenoid diets. Lines of least squares are shown for each diet treatment group (high carotenoids: (more ...)
In pairwise choice tests, females exhibited a significant preference for males on the high-carotenoid diet (mean±s.e. proportion of time spent with the high-carotenoid male: 0.74±0.05; one-sample t-test against a test mean of 0.5 (no preference): t16=5.25, p<0.001; ). Moreover, there was a significant positive correlation between the proportion of time that a female spent with the high-carotenoid male and the difference in subsequent longevity between two males that it saw (F1,15=5.30, p=0.036; ), suggesting that females were preferring to associate with males which ultimately lived longest.
Figure 4 The relationship between female preference (the proportion of time spent with the high-carotenoid male) and the difference in subsequent longevity between the high- and low-carotenoid males it saw. The dashed line represents no preference. The line of (more ...)